The field of this invention comprises apparatus and a method for the manufacture of printing plates which are to be used in lithographic offset printing.
Specifically the invention is concerned with the direct manufacture of lithographic printing plates using electrostatic techniques at high speed. The plates produced by the apparatus and method of the invention are superior to printing plates made by known methods whether photographically or electrostatically.
Lithographic printing plates made by the conventional photographic methods are expensive and complex; plates made by electrostatic methods which are known have never been widely used because they require considerable time to produce, have very little life and are low in resolution and spectral response. The latter two disadvantages are characteristic also of photographically made lithographic plates.
Lithographic printing is a process which is basically very old and well-known. For many years, even well into this century, the technique was practiced on special stone surfaces. A greasy image was applied to a surface, the non-imaged portions being rendered hydrophilic (water attractive, oil repellant). The imaged parts being hydrophobic (water repellant, oil attractive) when a paper receptor is pressed against the surface which has been wetted with water and the greasy ink, the greasy ink having adhered only to the image will be transferred to the paper.
In this process, since the only difference between the imaged and non-imaged areas is the presence of ink on the imaged areas, there is substantially no difference in height between the two areas, this type printing also being known as planographic. In this case of the classic method of letter press printing the imaged areas are in relief above the non-imaged areas or intaglio, that is engraved below the non-imaged areas.
Offset lithography is probably the most important method of printing today. The principle is that ink is offset first from the plate to a rubber blanket and then from the blanket to the paper receptor. There may be an intervening metal drum instead of a rubber blanket. When the printing plate is made, the printing image is rendered hydrophobic, i.e., repellant to water but also attractive to grease. The non-printing areas are rendered just the opposite, that is, hydrophilic. On the press the plate is mounted on a plate cylinder which, as it rotates, comes into contact successively with rollers wet by a water or dampening solution and rollers wet by grease-based ink. The dampening solution wets the non-printing areas of the plate and prevents the ink from wetting these areas. The ink wets the image areas which are transferred to the intermediate blanket cylinder. The paper picks up the image as it passes between the blanket cylinder and the impression cylinder.
In order to appreciate and understand the nature and advantages of this invention, one should comprehend the problems which are a necessary adjunct to the manufacture of a lithographic plate by conventional methods.
Offset plates of conventional construction of the type expected to make many thousands of impressions are expensive to manufacture. Ink receptivity is accomplished by using inherently oleophilic (having an affinity for oil) resins or metals like copper or brass on the image areas. Water receptivity of the non-image areas is usually achieved by using hydrophilic metals like chromium, aluminum or stainless steel and this receptivity is maintained in platemaking and storage by using natural and synthetic gums such as for example, gum arabic.
All offset printing plates which are used for long runs exceeding several thousands of impressions are made by indirect imaging methods. The copy or intelligence is first required to be photographed onto silver halide film and the film negative then used to transfer the image to the printing plate. The transfer is accomplished in all such cases by means of photographic projection onto a coating which is light sensitive and carried by the plate. The negative is used to project the image onto the plate and the processes which follow for the development of the image on the plate vary. Thus, the plates are required to be stored in darkness until used or the light-sensitive coating applied just before use. This is true of the three types of long-run offset plates which are most popularly used today.
The three types of long run plates which are known at this time are surface, deep etch and bimetal. The surface plates are those in which a light-sensitive coating is exposed to a negative, developed etc. The process of achieving the plate requires many steps and treatments. On deep etch plates, after exposure to the negative, the coating in the image areas is removed and coppered chemically and/or lacquered and inked so they are ink receptive. The plate is usually aluminum and the process is quite involved and requires considerable skill. Bimetal plates are similar to deep etch in that the light sensitive coating is removed from the image areas but these areas consist of copper or brass.
By reasons of the planographic nature of lithographic plates, electrostatic techniques would seem to lend themselves to the making of such plates. The toned images on a receptor or a photoconductive surface would seem to form the basis for hydrophobic and hydrophilic areas, respectively, but until the invention of the plates of the two copending applications, this has not been realized. Prior attempts, as for example using the zinc oxide electrophotographic member have not been successful.
Among the difficulties encountered have been low sensitivity, low resolution, mediocre quality, slow speed of manufacture, inability to stand up to wear on a printing press and limited chromatic response. Even conventional lithographic plates are normally exposed only with ultraviolet light and have limited chromatic response. Text and graphics must be separately produced on the plate by complex methods.
Given a metal or polyester based lithographic printing plate of the character described in the copending applications in which the photoconductive coating is as disclosed in U.S. Pat. No. 4,025,339, most, if not all of the disadvantages of prior lithographic printing plates are overcome. There is, however, another group of problems which must be solved in order to make such plates available for use in modern printing shops.
The plates of the copending applications require uniform charging over large areas, exposure over these same areas, toning and fixing. The exposure must be capable of accomplishment in a reliable manner by one who is not necessarily skilled in electrostatic techniques. It must be done routinely, with despatch. The apparatus used must handle copy originals conveniently and the processes to be effected must be done in such a manner that the many attributes of the plates of the copending applications are achieved.
Further, the apparatus must be simple, occupy little space, and produce uniform results every cycle of its operation.
These, then, are the ends sought by the invention, along with other advantages to be detailed and pointed out hereinafter. For example, the apparatus and method of the invention are capable of being used to make a metal or a polyester based lithographic printing plate in the same machine or one which is readily modified to produce one or the other.